1. Field of the Invention
The present invention relates to a reference voltage generator.
2. Description of Related Art
A liquid crystal driver for a mobile device such as a cellular phone is increasingly integrated into an IC that drivers an LCD panel, which is called one-chip integration, with the current trend towards downsizing and cost reduction. Further, a power supply circuit necessary for driving liquid crystals in an LCD is also increasingly built into a driver IC. In such a built-in power supply, a reference power supply has a function of determining a liquid crystal driving voltage. Therefore, if an output voltage of the reference power supply is unstable, LCD display quality is adversely affected. Accordingly, the stability of an output voltage of the reference power supply has been regarded as being particularly important recently.
As the reference power supply in a driver, a band-gap reference circuit (which is referred to hereinafter as a BGR circuit) that outputs a constant voltage in which temperature characteristics are cancelled is generally used. The BGR circuit is generally configured in such a way that a resistor is connected to each of two diodes in pairs with a different size ratio or the like. The BGR circuit then stabilizes the balance of two specific node potentials respectively connected to the diodes in pairs and further selects a certain resistance value, thereby canceling the temperature characteristics of the diodes and enabling output of a constant stable voltage. The BGR circuit is widely used as a basic voltage of a general IC.
Further, there is a strong demand for power saving during standby in a mobile device today. For example, low power consumption in standby mode when display is in the off state but operation in an IC is in the on state is a necessary feature of a device.
FIGS. 8A and 8B are exemplary schematic block diagrams of a display driver 1. The display driver 1 includes a BGR circuit 2, amplifiers 3 and 4, a driver amplifier 5, an LCD panel 6, and a logic circuit unit 7. FIGS. 8A and 8B schematically show the configurations in normal operation mode and in standby mode, respectively.
The BGR circuit 2 serves as a power supply generation reference for driving of the LCD panel 6 in normal operation mode. The highest level or the lowest level of a driving voltage of the LCD panel 6 (the higher level or the lower level of a gamma voltage) is determined based on the reference voltage. Therefore, the stability of the voltage is extremely important in order to prevent degradation of panel display quality. The amplifiers 3 and 4 amplify a voltage supplied from the BGR circuit 2 at a predetermined multiple number. The driver amplifier 5 drives the load of the LCD panel 6 by using a voltage from the amplifier 3 as a power supply voltage. The logic circuit unit 7 includes logic circuits 7a and 7b. The logic circuits 7a and 7b perform specified logic operation by using a voltage from the amplifier 4 as a power supply voltage.
On the other hand, in standby mode, display of the LCD panel is turned off, and the amplifier 3, the driver amplifier 5 and the LCD panel 6 enter the off state. The logic circuit 7b also enters the off state. However, the logic circuit 7a is still in operation because state setting after standby release, writing of display information from an external microcomputer or the like is performed, for example. Accordingly, the BGR circuit 2 and the amplifier 4 remain in the on state in order to supply a power to the logic circuit 7a. Because the BGR circuit 2 consumes a current during the on state, standby time is significantly degraded unless lowering an operating current as much as possible. It is thus an important feature of the BGR circuit 2 that current consumption is as low as possible.
As described above, it is becoming a necessary feature of the BGR circuit as a reference power supply for a liquid crystal driver to satisfy two demands: stability during normal operation and low power consumption during standby. Further, it is required to achieve the demands without increasing the circuit scale as a driver for use in a mobile device.
An example of a BGR circuit according to prior art is disclosed in Japanese Unexamined Patent Publication No. 2005-339724. FIG. 9 shows a BGR circuit 10 described in Japanese Unexamined Patent Publication No. 2005-339724. The BGR circuit 10 includes resistors R1 to R3, diodes D1 and D2, an operational amplifier OP1, and a PMOS transistor TP1. The resistor R1 and the diode D1 are connected in series between a reference voltage output terminal Vref and a ground voltage terminal GND. The resistors R2 and R3 and the diode D2 are connected in series between the reference voltage output terminal Vref and the ground voltage terminal GND. The operational amplifier OP1 has an inverting input terminal connected to an intermediate node A1 between the resistor R1 and the diode D1, and a non-inverting input terminal connected to an intermediate node A2 between the resistors R2 and R3. The PMOS transistor TP1 has a source connected to a power supply voltage terminal VDD, a drain connected to the reference voltage output terminal Vref, and a gate connected to an output terminal of the operational amplifier OP1. Note that, the symbols “VDD”, “GND” and “Vref” of the terminals designate the respective terminal names and also designate a power supply voltage VDD, a ground voltage GND and a reference voltage Vref, respectively, for the sake of convenience.